Download Characterization of Acoustic Transients: Calibration and

Characterization of Acoustic
Transients: Calibration and
Standardization
Robert Burkard
University at Buffalo
18 November 2011
Why quantify/verify/calibrate acoustic
signals?
-To relate percept or physiologic response
to acoustic stimuli- as we cannot directly
measure ‘hearing’, which is subjective.
-So we can compare results across
clinics/laboratories. What if all audiometers
were calibrated to differently. How would we
compare an audiogram from San Diego to
that from Buffalo?
Let us start with a definition of Sound
Pressure Level (SPL)
From ANSI S3.20-1995 American National
Standard Bioacoustical Terminology:
“Ten times the logarithm to the base ten of the
ratio of the time-mean-square of a sound, in a
stated frequency band, to the square of the
reference sound pressure in gases of 20 µPa.
Unit, decibel (dB); abbreviation: SPL;”
dB SPL = 10log(P2/20µ Pa2)
or, more familiarly: dB SPL = 20log(P/20µ Pa)
Where this comes from (Speaks, 1996):
A Bel is the log of the ratio of two sound powers
(in Watts), or sound intensities (in Watts/m2, or
Watts/cm2):
Bel = logI1/I2
and a tenth of a Bel (or deciBel, dB) is:
dB = 10log(I1/I2)
Some data many years ago suggested that the
lowest human threshold was near 10-12 W/m2, so:
dB IL = 10log(I/10-12 W/m2)
Again from Speaks (1996):
I = Prms2/ρos
Where I is sound intensity, P is pressure, and ρo is ambient density of the
medium, and s is the speed of sound in the medium; ρos is known as
characteristic impedance of the medium.
As this will factor out later, let us drop characteristic impedance:
I ~ Prms2
Also, at room temperature and at sea level,
10-12 W/m2 is equal to 20 µPa; substituting:
dB IL = 10log(I/10-12 W/m2)
dB SPL = 10log(P2/20 µPa2)
dB SPL = 10log(P/20 µPa)2
dB SPL = 20log(P/20 µPa)
Another definition from ANSI S3.20-1995:
Peak frequency-weighted sound pressure level:
‘Greatest instantaneous value of standard frequencyweighted sound pressure level, within a stated time
interval. Unit, decibel (dB),’
So peak SPL is:
pSPL = 20log(Pp/20 µPa), where Pp is peak Pressure
Relevant to this presentation, the crest factor of a signal
is the ratio of its peak pressure to its rms pressure. For
a sine wave, the crest factor is 1.414, or
20log1.414 = 3 dB
Most measure SPL with a sound level meter. When
using either a TDH-39, -49 or -50, or an, e.g.,
Etymotic ER3A insert earphone, this measurement
system is comprised of:
A coupler (6 cc or 2 cc), a type I measurement
microphone (typically condenser), an input amplifier,
filtering circuit, output amplifier, and some form of
display circuit.
Although the 6 cc and 2 cc couplers cannot be
considered artificial ears, they are commercially
available, and allow similar measurements to be
made in different locations.
from Curtis and
Schultz, 1986
from B&K 1971, 1982
from Johnson, Marsh and Harris, 1998
Fast: 125 ms τ
Slow: 1000 ms τ
Impulse: 35 ms τ for signals increasing in level,
1500 ms τ for signals decreasing in level
Peak: instantaneous largest pressure; hold
B&K 1986
Filtering
from B&K Frequency Analysis,
1977
from Johnson, Marsh,
and Harris, 1998
For TDH-39, -49, -50 earphonesUse a 6-cc coupler (several
types); For Etymotic (ER3A)
earphones-use a 2-cc coupler
(also several types)
from B&K Data Handbook, 1982
from Haughton, 2002
Instead of SLM, can use a microphone, preamplifier,
conditioning amplifier
from B&K Data Handbook, 1982
from Johnson, Marsh,
and Harris, 1998
Condenser microphone
-Most require 200 V polarization voltage
-may require voltage amplifier, to increase voltage
-the larger the microphone (1”, ½”, ¼”, 1/8”), the
greater the sensitivity
-The smaller the microphone, the higher the upper
cutoff frequency
Sensitivity: mV/Pa:
46.8 mV/Pa
20log.0468 = -26.6
Microphone- flat to 2 kHz, up
to 1 dB more sensitive from
2-7 kHz, less sensitive above
7 kHz
Now that we know about SLMs and other sound level
measurement equipment, how do we accurately measure
the sound pressure level of a transient?
Some possibilities:
-Increase the rate of the transients, using the fast time
weighting
-Measure peak SPL, using the peak-hold mode, if the
SLM has this capability
-Measure peak SPL using a microphone, preamplifier, and
conditioning amplifier
-Use the peak-equivalent SPL procedure
-Increase the rate of the transients, using the fast time
weighting
If you increase click rate in the ‘fast’ meter mode, there is an increase in
SPL with increasing rate
from Burkard 1984
Peak SPL (pSPL)
If you have a SLM with a ‘peak hold’ capability,
Route the earphone through the SLM, set the ‘peak hold’,
and reset to obtain a new measure.
-Problem 1: Response is to the largest instantaneous sound pressurewhether signal or noise.
-Problem 2: The time constant should be as short as possible (several
tens of µs or less) for an accurate estimate of pSPL of clicks.
-Problem 3: ‘Impulse’ is not the same as ‘peak’Impulse: 35 ms exponential τ for sounds increasing in level over
time, and an exponential τ of 1500 ms for sounds decreasing in level
over time.
Peak SPL: with microphone, preamp,
conditioning amplifier and oscilloscope
Simply route the transducer through the
coupler, microphone, preamp, conditioning
amplifier to oscilloscope.
Two calibration/measurement methods:
1. Microphone sensitivity
2. Acoustic calibrator
Microphone sensitivity:
1. Measure peak voltage of transient
2. Using microphone sensitivity, convert measured
voltage to pressure
3. Use the dB SPL formula to convert to pSPL
Sensitivity: 50 mV/Pa
Measured voltage: 10 mV (peak)
50 mV/1 Pa = 10 mV/ X Pa;
10 mV/50 mV = X Pa/1 Pa
X = .2 Pa
dB pSPL = 20log (.2 Pa/.00002 Pa) = 80 dB pSPL
Be careful, many conditioning amplifiers have adjustable gain.
Same microphone:
Microphone sensitivity 50 mV/Pa
40 dB gain in conditioning amplifier
Measure 1 volt peak voltage for click:
40 dB = 20logX, X = 100x gain
1000 mV/100 = 10 mV
50 mV/Pa = 10 mV/X Pa; X = .2 Pa
dB pSPL = 20log(.02 Pa/.00002 Pa) = 80 dB pSPL
Or
50 mV/Pa = 1000 mV/X Pa; X = 20 Pa
dB pSPL = 20log(20 Pa/.0002 Pa) – 40
dB pSPL= 120 – 40 = 80 dB pSPL
Using Acoustic Calibrator (or Pistonphone)
1. Place calibrator on microphone
2. Record rms voltage
3. Relate pressure (or SPL) to voltage
Use same microphone: 50 mV/Pa sensitivity
Place 114 dB SPL calibrator, and measure 500 mV
114 = 20 log(P/.00002 Pa)
5.7 = log(P/.00002 Pa)
501187 = P/.00002 Pa
P = 10.02 Pa
500 mV/10 Pa = 50 mV/Pa
Peak-Equivalent Sound Pressure Level (peSPL):
i. Route output of the SLM to an oscilloscope
ii. Measure amplitude of the, e.g., click (Vp, Vp-p)
iii. Route sine wave into earphone, adjust level of sine
wave until Vp or Vp-p is equal to that measured in /ii/
iv. Read SPL on readout of SLM
If Vp-p is 2X Vp (undamped), then
dB peSPL and dB p-peSPL are
equal
If Vp-p is equal to Vp (critically
damped), then dB pSPL is 6 dB
greater than dB p-peSPL.
from Burkard and Secor, 2002
Relationship between several methods for
measuring level of transients
From Burkard 1984
A Comparison of Different Measures of
Sound Pressure Level (SPL) for Click
Stimuli in Both Supra-aural and Insert
Earphones
Robert Burkard
27 June 2011
(IERASG)
Introduction
IEC 60645-3 recommends using peak-to-peak peak
equivalent sound pressure level (p-p peSPL) for the
calibration of acoustic transients. RETSPLs of transients
recommended by ISO 389-6 are based on p-p peSPL.
A review of the literature reveals that calibration of
transients does include the use of p-p peSPL, but also
uses baseline-to-peak peSPL (b-p peSPL) as well as
peak SPL (pSPL). The relationship between b-p peSPL
and pSPL is fixed (that is, the crest factor of a sine wave:
20log1.414 = 3.02 dB).
However, the numerical relationship between p-p peSPL
and b-p pe SPL can vary over a range of 6.04 dB, making
it impossible to know the relationship between pSPL and
p-p peSPL without empirical measurements
dB p-p peSPL can range from 0 to 6 dB less than dB b-p peSPL
As Chair of ANSI S3/WG71 “Auditory Evoked
Potentials”, I have been struggling with how to include
the various measures of transient level (p-p peSPL, b-p
peSPL and pSPL) in the ANSI Standard, and yet still
harmonize with the international standard.
I made some acoustic calibration of both supra-aural
and insert earphones, using a variety of couplers and
microphones, to ascertain the relationship between p-p
peSPL and b-p peSPL. These values, for various
earphones and measurement systems may be
considered for inclusion as Annex material in the
forthcoming ANSI standard
Grass S88 to TDT current amplifier to earphone (and Tektronix o-scope)
Earphone to coupler to microphone to LD824 SLM to o-scope
Frequency responses: SRS Spectrum analyzer to current driver to earphone
to coupler/microphone/SLM
Peak Equivalent SPL
Measure rms voltage (Vref), AC output of SLM to 1 Pa acoustic calibrator (94 dB
SPL)
Measure bpV:
dB pSPL = 20log(bpV/Vref) + 94
dB b-p peSPL = 20log (bpV/Vref/1.414) + 94
Measure ppV:
dB p-p peSPL = 20log(ppV/Vref/2.828) + 94
A
bpV
ppV
bpV
ppV
B
C
IEC 318 #2 (BB); 2 cc couplers: HA 2 1” and ½”, Occluded ear simulator
NBS 9A
IEC 318 TM
HA-2, 2 cc coupler, 1” microphone
2 cc, HA-2, ½” microphone
HA-1, 2 cc, ½” microphone
Occluded Ear Simulator, with rigid tube
Occluded Ear Simulator
Foam
Insert Earphones:
6 ER-3A from Etymotic (thank you): 2 10 ohm, 2 50 ohm, 2 300 ohm (soft tube)
7 EAR insert earphones: various impedances, both soft and more rigid tubing
Supra-aural Earphones:
6 TDH-39s, 3 TDH-49s, 3 TDH-50s (various impedances)
Measurements:
For pSPL/peSPL: Adjust voltage level until pSPL on SLM is ~100 dB
Measure b-pV and p-pV. Redo 3 times, for each coupler and earphone
Calculate pSPL, b-p peSPL, p-p peSPL from voltage measurements, and
calculate mean values
Frequency response: Supra-aural: NBS-9A, LD 1” mic: Constant voltage to
all earphones. For inserts: HA-2 (BB) TM 1/2” mic: Adjust voltage so that
near 1000 Hz, ~105 dB SPL. Vary frequency at this voltage, in 1/3 octave
steps from 100-~10,000 Hz.
Constant voltage (NBS-9A, 1” mic uncorrected)
TDH-39
TDH-49
115
110
110
39-1
39-3
100
39-4
95
dB SPL
dB SPL
105
39-2
105
49-1
100
49-2
49-3
39-5
95
39-6
90
85
90
100
1000
10000
100
Frequency (Hz)
TDH-50
10000
Earphone Comparison
110
110
105
105
50-1
100
50-2
50-3
95
dB SPL
dB SPL
1000
Frequency (Hz)
TDH-39
100
TDH-49
TDH-50
95
90
90
100
1000
Frequency (Hz)
10000
100
1000
Frequency (Hz)
10000
Etymotic versus EAR: constant SPL @ 1008 Hz
Insert 11 - 17
Insert 1 - 6
120
120
110
Insert-4
80
Insert-5
Insert-6
70
Insert-13
90
Insert-14
Insert-15
80
Insert-16
70
Insert-17
60
60
100
1000
10000
100
100000
1000
10000
Frequency (Hz)
Frequency (Hz)
Insert Comparison
120
110
100
dB SPL
dB SPL
Insert-3
90
Insert-12
100
dB SPL
Insert-2
100
Insert-11
110
Insert-1
Insert 1 - 6
90
Insert 11 - 17
80
70
60
100
1000
10000
Frequency (Hz)
100000
100000
Peak SPL (oscilloscope) – Peak SPL (SLM)
Supra-aural Earphones
NBS-9A (BB) mean
min
max
TDH-39
-0.63
-1.77
-0.10
TDH-49
0
-0.3
+0.3
TDH-50
-0.4
-0.03
-0.6
NBS-9A(TM) mean
min
max
-0.51
-1.35
+0.1
+0.06
-0.05
+0.13
-0.43
-0.18
-0.58
IEC318 (BB) mean
min
max
-0.37
-1.63
+0.2
+0.14
-0.13
+0.37
-0.34
-0.30
-0.47
IEC318 (TM) mean
min
max
+0.56
+0.30
+0.80
+0.38
+0.17
+0.57
+0.40
+0.30
+0.47
Baseline-to-Peak peSPL – Peak-to-Peak peSPL
Supra-aural Earphones
NBS-9A (BB) mean
min
max
TDH-39
3.15
2.7
3.87
TDH-49
3.94
3.30
4.47
TDH-50
5.24
5.17
5.33
NBS-9A(TM) mean
min
max
2.40
1.80
2.68
2.90
2.48
3.30
3.87
3.75
4.00
IEC318 (BB) mean
min
max
3.50
2.77
3.83
3.38
3.03
3.67
4.00
3.53
4.30
IEC318 (TM) mean
min
max
3.39
2.77
3.80
2.82
2.33
3.27
3.78
3.53
4.03
Peak SPL (oscilloscope) – Peak SPL (SLM)
Insert Earphones
HA-2 1” BB mean
min
max
Etymotic
+0.33
+0.27
+0.40
EAR
+0.29
+0.20
+0.40
Etymotic
HA-1 1/2”(BB) mean +0.03
min
-0.03
max +0.07
EAR
+0.08
+0.03
+0.13
HA-2 1” TM mean
min
max
+0.28
+0.23
+0.33
+0.28
+0.23
+0.30
Occluded Ear mean +0.09
Sim- BB-rigid min
+0.03
tube
max +0.17
+0.12
+0.10
+0.17
HA-2 ½” BB mean
min
max
+0.22
+0.13
+0.37
+0.19
+0.13
+0.27
Occluded Ear mean +0.17
Sim- BB-foam min +0.10
max +0.23
+0.19
+0.10
+0.30
HA-2 ½” TM mean
min
max
-0.09
-0.20
0
-0.40
-0.73
-0.16
Baseline-to-Peak peSPL – Peak-to-Peak peSPL
Insert Earphones
HA-2 1” BB mean
min
max
Etymotic
2.69
2.60
2.80
EAR
2.59
2.23
3.00
Etymotic
HA-1 1/2”(BB) mean 4.66
min
4.27
max
4.90
EAR
4.45
3.63
5.07
HA-2 1” TM mean
min
max
2.61
2.37
2.77
2.59
2.23
2.97
Occluded Ear mean 1.23
Sim- BB-rigid min
1.17
tube
max 1.37
1.15
.90
1.43
HA-2 ½” BB mean
min
max
2.45
2.37
2.57
2.35
1.93
2.73
Occluded Ear mean 1.49
Sim- BB-foam min
1.20
max 1.67
1.59
1.17
2.00
HA-2 ½” TM mean
min
max
2.32
2.10
2.60
2.20
1.70
2.63
Summary: pSPL/peSPL measures
1. pSPL as measured by the LD 824 SLM using the peak hold mode
(Flat) typically produces a click level within several tenths of a dB
from that observed using the AC output of the SLM, using
procedures similar to those used to measure peSPL.
2. For the supra-aural earphone measures, there was a substantially
greater range of differences between the two pSPL measures, as
compared to that observed for the insert earphones.
3. Sources of differences between the two measures may include:
a. The integration time required for the SLM peak-detector circuit, which
would lead to a lower SPL in the peak-hold value of the SLM.
b. Spurious noise in the room or from the earphone. This could lead to
a high-level transient that could produce a value from the SLM peakhold circuit that exceeds that using the peSPL approach.
4. The difference in peSPL using the p-p and b-p
approaches varies across earphone type, coupler
used, and microphone used. There is also some
variation across individual transducers within a
specific type of transducer.
5. Thus, to convert from b-p peSPL to p-p peSPL, or
vice versa, one must have conversion values for all
transducers used for AEP clinical efforts, and for all
couplers (and possibly earphones) used to make pe
SPL measurements.
As couplers do not provide an exact SPL measurement at the
tympanic membrane: How about ‘real-ear’ measures?
Speaks 1996
The problem with ‘real-ear’ measures is that at frequencies
above a few kHz, there are standing waves in the ear
canal, and the sound pressure measurement at the probe
microphone may not accurately reflect the SPL at the
plane of the TM
Other Calibration Issues for Transients:
Spectral Analyses:
In most cases, we will route the output of the microphone/amplifier
or SLM to a digital spectrum analyzer.
We will often low-pass filter the input to spectrum analyzer (an
antialiasing filter; may be included in the spectrum analyzer)
For continuous signals (like tones), we will typically pass this
through a windowing function, like a Hanning or Flattop window.
This ‘window’ attenuates the response at the onset and offset, to
prevent spectral leakage
For transients, should use a Uniform or Rectangular window
(equal weighting), and capture entire transient in the time window
of analyzer, to prevent unintentional changing of the spectrum of
the transient
Why are ‘clicks’ typically 100 µs pulses for humans?
from Durrant 1983
from Burkard 1984
Linear/Log spectrum: envelope effects
from Burkard 1984
Toneburst
duration
bandwidth (Hz)!
500 Hz
1000 Hz
2000 Hz
4000 Hz
5 ms
5 ms
5 ms
5 ms
410-610 Hz: 200 Hz
905-1105 Hz: 200 Hz
1903-2103 Hz: 200 Hz
3901-4101 Hz: 200 Hz
.57
.288
.144
.072
500 Hz
1000 Hz
2000 Hz
4000 Hz
4 ms
2 ms
1 ms
0.5 ms
390-640 Hz: 250 Hz
780-1280 Hz: 500 Hz
1562-2562 Hz:1000 Hz
3123-5123 Hz: 2000 Hz
.71
.71
.71
.71
!: CF = (Fl x Fu)0.5
#octaves*
* #octaves: (log{Fu/Fl})/log2
Should you use a constant time envelope or a constant
number of cycles envelope across toneburst
frequency?
1. The cochlea is scaled logarithmically (~5
mm/octave) in at least the mid- to high-frequencies. If
you wanted to stimulate a constant proportion of the
cochlear partition, a constant number of cycles
(producing a constant number of octaves) is
preferable.
2. Using constant-slope stimuli, Suzuki and Horiuchi
(1982) found that the ABR is elicited by the first 1-2
cycles of the envelope- again supporting the notion of
using a constant number of cycles (perhaps a 2-1-2
cycle rise-plateau-fall time)
Some Final Thoughts:
-Reports often are vague about calibration methods used
of acoustic transients. pSPL can range from 3 to 9 dB
greater than peSPL.
-In all cases, you should state the instrumentation and
technique used to determine the SPL of transients.
-In my opinion, it is best to use the baseline-to-peak
peSPL method, as it can easily be converted to pSPL (by
adding 3 dB). To perform a similar conversion using
peak-to-peak peSPL requires knowledge of the impulse
symmetry. However, the current international standard
recommends the peak-to-peak peSPL approach (IEC
645)
-We should state, in all reports, the normative population
used to determine a 0 dB nHL value for transient stimuli
(i.e., the RETSPL). This must include the rate at which
the stimuli are presented (temporal integration affects
perceptual threshold but not AEP threshold), and subject
characteristics (including audiometric criteria). Some
quantification of the ambient noise conditions of of test
room is useful.
-Although ISO 389-6 has provided RETSPLs for clicks
and ‘standard’ tonebursts, there is no equivalent ANSI
document for RETSPLs of transients. Thus, it is
imperative that we quantify our signals as precisely as
possible. It is likely that 35 dB nHL values can vary by
more than 5 dB across labs (or clinics), and this 5 dB
difference will likely have a substantial influence on
pass/fail rates in infant screening programs.
These calibration issues are even more problematic in AEP
studies in rodents (such as mice)
-The earphones used for humans are limited in bandwidth,
as mouse hearing often extends well above 40 kHz
-A 2-cc coupler does not represent a reasonable
approximation to the volume (or the input impedance) of a
mouse ear
-A 1” microphone starts rolling off below 10 kHz, and smaller
microphones (e.g., ¼” or 1/8”) are preferable
-Often a SLM is designed for the human audiofrequency
range (up to 20 kHz), and attenuates signals above this
range
We cannot directly measure sensation, and
to determine hearing ability, we must pair
the signal with the response.
Those who use AEPs to evaluate the
hearing of, e.g., newborns, know the
challenges of recording a repeatable AEP
in a clinical environment. It is equally
important to pay attention to the acoustic
stimulus used in these evaluations, as this
will affect of results of your hearing
evaluation.
Questions?